1. Field of the Invention
The present invention relates to a device for indicating the exchange time of the drum unit used for image forming apparatus.
2. Description of the Prior Art
In the cleaning process of electrophotography applied to such apparatus as copying machines and laser printers, it is so arranged that the toner remaining on the surface of the photosensitive drum (hereinafter referred to as drum) is scraped off with blades of a cleaning device and is recovered into a waste toner tank.
It is, therefore, necessary to discard the waste toner when the waste toner tank is filled fully with recovered waste toner.
While the cleaning blades used for the cleaning process have a limitation on the usable life because of wear due to continuous use.
Life of the drum itself is also limited as the sensitivity is lowered and the charging property is deteriorated through continuous use.
Under the situation, an increasing number of image forming apparatus are made to the type for which the drum of a limited life, the cleaning blades, and the waste toner tank that needs periodical discharging of the waste toner are integrated into a drum unit, and the drum unit is assembled into the image forming apparatus for easier maintenance.
For the image forming apparatus of this type, therefore, the whole drum unit is to be exchanged at certain time to maintain the performance.
In an image forming apparatus, for example, the drum unit is replaced when the waste toner tank is filled with waste toner up to certain level regarding the time as the life of the drum unit. As the means to indicate the time of replacement, the system in which further printing is disabled so that the operator is forced to change the drum unit is employed.
This system, however, is very inconvenient for the operator as printing is disabled all of a sudden and exchanging is impossible without a spare drum unit.
By the apparatus disclosed in U.S. Pat. No. 4,551,000, as another example, the drum unit is to be exchanged when drum revolution reaches certain limit, and as the means to indicate the time of exchange, it is proposed to turn a color coded disc or to glow a lamp.
As this arrangement, however, is merely to indicate the life of the drum unit, printing is continued by the operator regardless of the indication in most cases, which allows possibility of later troubles.